The described below is a micromechanical system having at least one beam-shaped element, which has an exposed end and is connected at its other end to a further element of the micromechanical system.
Micromechanical systems, which are often also referred to as micro systems or MEMS (micro electro mechanical system), are becoming increasingly popular in particular on account of their small size, their comparatively low price and also their high reliability. This relates for example to the use of micromechanical systems as sensors, for instance in the form of sensors for the detection of acoustic emissions, solid-borne sound sensors, acceleration sensors and tilt sensors, angular rate sensors or pressure sensors.
Acoustic emissions (AE) are normally understood to be a phenomenon wherein elastic waves are generated by an impact excitation resulting from a sudden release of energy within a solid body. Corresponding acoustic emission signals, which propagate in the form of structure-borne sound in the solid body, normally occur in a frequency range of about 20 kHz up to about 1 MHz. In this situation, acoustic emission signals exhibit a high sensitivity with regard to mechanical damage to a solid body or to an object. Micromechanical sensors for the detection of acoustic emissions (acoustic emission sensors) are therefore employed in particular for monitoring the wear of mechanical components such as roller bearings for example. In this situation, corresponding sensors normally have a system capable of vibration having a seismic mass suspended or secured on spring elements. External forces or accelerations cause a deflection of the seismic mass with respect to a fixed suspension. This relative movement is evaluated, whereby a capacitive principle is often applied for signal recovery. In this connection, the seismic mass includes electrode arrangements which can for example be implemented like a comb and together with a fixed counter electrode form a variable capacitance. In this connection it is possible to detect acoustic emissions by sensing the value of the capacitance or any change therein. In this situation, it is assumed as a general rule that the seismic mass itself and also the electrode arrangements form inherently inflexible elements in the respective usable frequency range.
There is currently a trend towards ever higher natural frequencies of micromechanical systems. This applies both to wideband applications and also to resonant systems. In addition to the sensors already mentioned for the detection of acoustic emissions, micromechanical filters and mixers for the high-frequency range may be quoted here as further examples.
Micromechanical systems are manufactured for example by dry etching processes employing surface and near-surface MEMS technology. As a general rule the structures or systems thus manufactured have a high aspect ratio (HAR). In this situation, such a high aspect ratio of the structures is normally required in order to minimize the cross sensitivity of the system and at the same time to increase the sensitivity in the movement direction.
One advantage of micromechanical systems of the aforementioned type lies in the capability for batch processing in the wafer and the low manufacturing costs resulting therefrom. In this context, the space requirement for the micromechanical system is a major cost factor because a smaller space requirement enables a higher chip density on the wafer.
On the other hand, to achieve a high sensitivity of micromechanical or microelectromechanical systems large surface area electrodes or electrode systems are often required. In order to obtain as large an electrode surface area as possible the surface of the electrode systems is often increased here by vertical structuring whilst the lateral dimensions remain unchanged, whereby the “lateral” or “vertical” particulars relate in each case to the wafer plane. This produces ramified electrode structures, for example. With regard to such electrode structures, there is however in practice a danger that the natural frequency of the micromechanical system, or of a system capable of vibrating of the micromechanical system, is reduced on account of the high seismic mass of the electrodes and on account of the suspension of the latter. This relates for example to the case where the electrodes have a so-called interdigital structure and the suspension of the electrodes is implemented by beam structures.
Generally speaking, there is a requirement that the electrode structures themselves exhibit a very high rigidity or a very high natural frequency so as to exclude any influence on the functioning of the micromechanical system. One way of guaranteeing this is the use of solid, large surface area carrier systems. This is however accompanied by the disadvantage that corresponding large surface area carrier systems have a large space requirement and thus counteract the previously described efforts directed towards a particularly cost-effective manufacture of corresponding micromechanical systems. A tapering for example of beam structures for the suspension of the electrodes, conceivable in principle, is as a general rule disadvantageous or not possible on account of the cubic reduction associated therewith in the rigidity of the structure. On the other hand, a widening of the beams in accordance with the above statements counteracts the efforts directed towards a low space requirement. Although a general reduction in size of the micromechanical system results in higher natural frequencies, this does however have the disadvantage here that a decrease in the sensitivity of the micromechanical system results at the same time.
The above statements and problems also apply with regard to further beam-shaped elements of the micromechanical system which have an exposed end and are connected at their other end to a further element of the micromechanical system. This also includes passive elements such as for example shielding electrodes or other structures in the form of beam-shaped elements which are suspended on one side at an anchor point. There is also a requirement here for these structures to exhibit a high rigidity and at the same time occupy as little as possible of the silicon surface area essentially determining the price of the micromechanical system.